1. Field of the Invention
This invention generally relates to methods and apparatus for controlling introduction of catalysts and/or catalyst additives into fluid catalytic cracking units (FCC units). Such introduction methods and apparatus are used to control the production of various primary and secondary products in a wide variety of catalytic reactions.
2. Description of the Prior Art
Fluid catalytic cracking processes are employed throughout the chemical and petrochemical industries. The need to constantly adjust such processes can be engendered by any number of unavoidable changes, e.g. changes necessitated by variations in: (1) the character of a feedstock, (2) more stringent pollution control mandates and/or (3) changed product quality requirements. The ability to closely adjust or otherwise control such processes also serves to minimize the use of, and hence the costs of, expensive catalysts and/or catalyst additive materials. Such control also serves to reduce the complexities associated with the often competing effects associated with the simultaneous use of several different kinds of catalyst materials.
In order to better appreciate the nature of the herein disclosed methods for introducing catalyst materials into a FCC unit, one first should envision its operation. Typically, a catalyst bulk inventory of tons, indeed even hundreds of tons, of catalyst materials flow (often at high velocities) through the fluidized beds, reaction zones, and regeneration zones which make up the unit. Next, it should be appreciated that several distinct kinds of catalyst may be distributed, preferably to a steady-state condition of homogeneity, in a circulating bulk catalyst. Each different catalyst species will, however, diffuse through the bulk catalyst mass and gradually be deactivated, attritted and elutriated at its own individual rate. For the most part these rates are determined by the hardness, durability, and density characteristics of each respective catalyst species. Each catalyst also contributes to a delay time which is such that, at some optimum rate and allowable composition, the overall bulk catalyst composition settles down to a continuous rhythm of steady-state performance with respect to introduction of each catalyst material, much as a human being will reach a fairly constant average level of medication by ingestion of fixed dosages of different drugs taken at fixed times.
Bulk catalyst blends often contain a major fraction of a primary catalyst and a minor fraction of one or more catalyst additive materials. It is also common practice to place more than one catalyst species in a single catalyst particle. In their more complex forms, individual catalyst particles are usually comprised of an inert matrix or binder material which serves to hold two or more different catalyst species together in a single overall binder/catalyst matrix. Nonetheless, for the most part, a given FCC function is normally accomplished through the use of a given catalyst species. In other words, a specific catalyst species is normally used to catalyze a given type of catalytic reaction regardless of whether or not a given catalyst particle contains a single catalyst species or more than one catalyst species.
Those skilled in this art also will appreciate that most catalysts used in FCC units are made in the form of so-called microspheroidal particles (MS particles). Such particles are specifically designed to be placed in a "fluidized state" by entraining them in high velocity vapor streams. Consequently, catalyst particles which are to be placed in a fluidized state must conform to rather narrow ranges of particle size and density limitations which are necessary to achieve fluidization.
By way of further explanation of the context in which this invention resides, it also should be noted that a typical FCC unit is normally operated with a view toward simultaneously attaining and maintaining several, performance parameters. For example, a petroleum refining FCC unit may be called upon to convert a petroleum feedstock into gasoline of a given octane rating while simultaneously holding production of a pollutant such as SO.sub.x to a prescribed level. In this case the petroleum cracking function is carried out by a bulk catalyst while the SO.sub.x control function is accomplished through the use of a so-called catalyst additive material. Other common performance parameters which an operator may wish to control in a petroleum FCC unit might include (but not be limited to) product yield(s), coke lay down (coke deposit on the catalyst) and gas make (control of the nature and relative proportions of chemical components of the end products--with a particular view toward avoiding or limiting production ethane, ethylene and hydrogen. The attainment of each such performance parameter also may be thought of as a function being performed by the FCC unit. It also should be noted that a given catalytic function being performed by a FCC unit may be a secondary, or even undesirable, function. In many, if not most, catalytic cracking processes, a bulk catalyst will be used to carry out a primary function (e.g., production of gasoline from a petroleum feedstock) while a catalyst "additive" is simultaneously used to carry out a secondary function (e.g., reduce SO.sub.x emissions).
It also should be noted in passing that it will very often be the goal of the methods taught by this patent disclosure to inject a catalyst additive into a FCC unit in a manner calculated to maintain a given concentration of the additive in the unit in order to control fluctuation in the FCC unit's performance with respect to some function (e.g., SO.sub.x production) to some prescribed level or range. To a large extent, the introduction of catalyst additives in order to attain and maintain desired concentrations of given catalysts has been through the use of rather imprecise --indeed, even haphazard--methods. For example, bulk catalyst suppliers may simply add certain catalytic additive(s) to their bulk or primary catalyst products in order to satisfy their customer's catalyst additive needs. At best, this practice represents an unwelcome diversion from the bulk catalyst supplier's business since the bulk catalyst ingredient(s) normally represents most (e.g., 80-99%) of most bulk catalyst/catalyst additive products.
It also should be noted that a typical catalyst additive material is usually much more expensive than a typical bulk catalyst. Hence, addition of a catalyst additive to a bulk catalyst usually increases the costs of the overall product. These added cost considerations also are amplified by the fact that a bulk catalyst supplier is usually forced to add much more of a given additive to a bulk catalyst/additive mixture than is normally required by the FCC operator since it is imperative that the overall catalyst material be able to perform in a wide range of operating circumstances. For example, it is well known that consumption of most catalyst additive materials can vary considerably with relatively small changes in the quality of a petroleum feedstock, e.g., small changes in its content of sulfur, heavy metal contaminants, etc. Consequently, given an expectation of some changes in the character of a given feedstock, a bulk catalyst supplier will usually "overdose" a given bulk catalyst with each catalyst additive placed in the bulk catalyst.
Unfortunately, such overdosing practices can sometimes create undesired fluctuations in a FCC unit's performance with respect to other catalytic function being carried out by the bulk catalyst and/or by a different species of catalyst additive. Consequently, many FCC operators prefer to introduce catalyst additives at the FCC unit according to their more particular local needs. Those skilled in this art also will appreciate that bulk catalysts are often better added on a more or less "continuous" basis, while catalyst additives tend to be needed on an "intermittent" basis. That is to say that, even though it might seem that it would be best to simply introduce each catalyst additive species continuously along with the bulk catalyst at a known rate in order to maintain the additive's relative proportion in the overall bulk catalyst, this is not usually the way such processes are carried out. In the real world of "cat cracking" such constancy is rarely achieved and, in many cases, is not even desired.
Consequently, a variety of procedures and mechanical systems for "on-site" addition of fresh catalyst materials to FCC units have been tried with varying degrees of success. Some of the problems which heretofore have been experienced with such on-site catalyst additive introduction procedures and/or apparatus follow from the fact that most prior art catalyst introduction systems which are employed to introduce catalyst additives utilize large segments of the mechanical systems which are normally used to introduce the bulk catalyst material into the FCC unit. For example, the apparatus most commonly employed for addition of catalyst additives to FCC units are those so-called "lift pipe" systems which are normally used to carry relatively large quantities of bulk catalyst from a storage hopper to a FCC unit in a high velocity stream of air in which the catalyst particles are entrained. Other less common, but still technically viable, catalyst additive introduction systems are based upon use of such varied devices as "star" type feeders, addition pots and pinch valves. A wide assortment of "home made" systems also are found throughout the chemical and petrochemical industries. Their use is often brought about by frustration with the use of existing bulk catalyst addition systems for the purpose of introducing catalyst additive materials.
There are several reasons why the systems used to introduce bulk catalyst are not well suited to the introduction of catalyst additives. For one thing, bulk catalysts usually are introduced in large amounts, often on a continuous basis, while catalyst additives are usually introduced in much smaller amounts, usually on an intermittent basis. Moreover, and regardless of the relative amounts of catalyst being delivered, the amount of catalyst introduced by such addition systems is "inferred" by the length of time an air stream is allowed to carry a catalyst material--regardless of its identity. That is to say that, regardless of whether or not a bulk catalyst material or a catalyst additive is being handled, a clocked metering device is used to control an air stream according to some predetermined time schedule. Certain other problems follow from the fact that delivery of catalyst additives to the FCC unit usually takes place via a lift pipe which is permanently "sized" with a view toward continuously delivering relatively large amounts of bulk catalyst. This circumstance often creates large errors when the bulk catalyst delivery system is called upon to transfer relatively small amounts of catalyst additives to a FCC unit--especially on an intermittent basis.
In either case, however, an FCC operator simply will assume that operation of the air stream which entrains catalyst particles (of whatever type), for a given period of time, will carry a given amount of catalyst particles to the FCC unit. For example, an operator might assume that an air stream, at a given pressure (e.g., 50 psi) will deliver a given amount of catalyst through a given sized lift pipe at a given delivery rate (e.g., 4 tons per hour). Thus, a timer system and/or a human operator will assume that operation of the catalyst delivery air stream, for a given period of time (e.g., one-quarter of an hour), will deliver a given amount of catalyst in a given period of time (e.g., 1/4 hr..times.4 tons/hr.=1 ton, delivered in 15 minutes). Unfortunately, such precise delivery does not always take place in the "real world" of catalytic cracking, especially when a lift pipe which was originally sized to deliver say 40 tons of bulk catalyst per hour, on a continuous basis, is also called upon to deliver a catalyst additive at a much lower rate of say only 4 tons per hour on an intermittent basis.
Aside from the inappropriate sizing of the equipment, errors also are frequently produced by the fact that a plant's air supply system may, and often does, operate over a widerange of pressures, e.g. an "assumed" 50 psi air supply may, in fact, operate at pressures ranging from say 40 to 60 psi at any given point in time. Such pressure differences will cause the system to deliver differing amounts of catalysts in a given time period. Any errors resulting from such differences in pressure become more pronounced in the case of introducing relatively small amounts of catalyst additive on an intermediate basis. Thus, for any or all of the reasons noted above, and even though a given catalyst delivery system may operate for a precisely measured period of time (e.g., 15 minutes), a prescribed amount of catalyst (e.g., the 1 ton "assumed" to be delivered in the 15 minutes of our previous example) may not, in fact, be delivered to a FCC unit. Thus, under the influence of such unintended, undetected and/or cumulative operating errors, an FCC function which is being controlled by a given catalyst or catalyst additive eventually will operate outside a prescribed range with respect to some desired condition or performance level (e.g., SO.sub.x production, octane rating of gasoline, etc.) even though the feedstock is of a uniform quality. Obviously, changes in feedstock quality will serve to compound the effects of all such errors.
Those skilled in this art also will appreciate that many prior art methods for controlling introduction of catalyst additives are by no means "automatic". Indeed, many catalyst addition systems must be continuously monitored and controlled by FCC operations personnel even though the catalyst addition systems are placed on a mechanical timer which adds a nominal volume of fresh catalyst according to a predetermined schedule. Some catalyst addition control systems simply operate until the errors in the addition of the catalyst accumulate to a point where the FCC unit begins to operate beyond a given performance level, e.g., until it produces too much SO.sub.x, too little gasoline of a given quality, too much coke and so on. At such a point the board operator will "override" operation of a metered catalyst addition system and "manually" introduce or withhold catalyst additive until the operation of the FCC unit is brought back to some desired performance level with respect to a given performance parameter being controlled by a particular catalyst. Those skilled in this art also will appreciate that such responses by means of changes in catalyst addition rates are by no means instantaneous, indeed for larger FCC units several hours may be needed to effect such changes.
It also should be noted in passing that, when such manual intervention occurs, the all too "human tendency" for an FCC operator is to add much more of a given catalyst additive than is actually prescribed to correct a problem. The operator often "overreacts" in order to get through his work shift without any further problems. However, the operator also has to pay a price with respect to such overdosing practices since the overdosing of one catalyst component of a bulk catalyst may itself create competing demands or problems which cause frequent and sometimes severe distractions from the operator's other duties. Thus, for all of the above-noted reasons, there exists a need for improved methods and apparatus for introducing catalyst materials into FCC units.